Bongard Problem #16 (solution)

Left: Turning clockwise, from inner end
of line to outer end.

Right: Turning counter-clockwise, from
inner end of line to outer end.

Note: Whenever I presented this problem to people, I noticed
that moments before coming up with the solution invariably and
everybody uses their index finger to spatially trace the curves.
After performing a few quick clockwise and counter-clockwise
rotations with the finger, they verbalize their finding. There
must be something to this mind-movement coordination, which my
program will be unable to explore. I hope there is not any deep
cognitive principle hidden there.